Earth has plenty of life. Where else may it lie in the solar system? Some hope for Mars which is on the edge of the solar system Goldilocks zone. Others believe it lies on on of the other moons such as Titan, Enceladus or Europa. Ten times thicker than Earth's, Titan's atmosphere extends nearly 370 miles above its frigid surface. It's a literal chemical factory, where nitrogen and methane are zapped by the sun's ultraviolet rays and transformed into organic molecules, some of which descend to the moon's surface while others rise up above the clouds, creating a bluish high-level haze of hydrocarbons. Strange lakes and rivers of methane have been found on its surface.

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Titan has a variable geology, including active volcanoes, and weather patterns. Titan is thought to be roughly analogous to ancient Earth, albeit much colder. There is also some water ice on Titan. It has also been suggested that a form of life may exist on the surface, using liquid methane as a medium instead of water; and anomalies in atmospheric composition have been reported which are consistent with the presence of such a life-form, but which could also be due to an exotic non-living chemistry.

Enceladus has a plume escaping from its south pole that contains salt water vapor. This suggests that the moon has either a salt water sub surface ocean or salt water in caves. Enceladus also has an internal heat source, likely caused by a combination of tidal forces and radioactive decay. The presence of liquid water on Enceladus suggests that the moon may contain some kind of microbial life.

Europa's surface is composed of ice, underneath which is an ocean scientists believe is of liquid water. The water is heated by tidal forces, thus making Jupiter's moon another candidate for life beyond the Earth. Life, if any, in Europa's oceans would be similar to those in the deep oceans on Earth that have no access to sunlight.

Enceladus and Europa are moons with an ocean locked under their surface if it exists at all. Titan has a true and thick atmosphere.

The research paper, "Clues on the importance of comets in the origin and evolution of the atmospheres of Titan," by Trigo-Rodriguez and F. Javier Martin-Torres (Center for Astrobiology, Madrid, Spain), recently published in the journal Planetary and Space Science, offers insight into the atmospheric affinities of Earth and Titan.

"We see Titan as a natural oasis of remarkable astrobiological significance to understand the environment in which origin of life took place on Earth," Trigo-Rodriguez says. "It seems that a plausible scenario to build life consists of a dense atmosphere, where small particles like organic haze and meteoric metals could act as catalysts for the formation of more complex organic compounds from simple precursors such as carbon monoxide and methane, thus promoting increasing complexity."

The methane in the atmosphere may come mainly from Titan’s lakes of liquid hydrocarbon. But to really understand what is re-supplying the methane, Martin-Torres would like to see another probe sent to the moon’s surface. the methane must come from somewhere and be in balance for the present atmosphere to have formed.